Maintained by Robin Tecon, microbiologist and postdoctoral researcher at the Swiss Federal Institute of Technology Zürich. This blog is about bacteria (and other microbes) and the scientists who study them.

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Sunday, February 17, 2013

Coliforms and operational definitions

Recently, I was talking about coliforms
with my colleagues from the lab. ‘Coliforms’ is the name given to a group of bacteria that
usually serve as indicators of fecal contamination in water and food samples,
because most coliforms come from the intestinal flora of animals. For this
reason, water and food that contain too high levels of coliforms are deemed unfit
for human consumption (the US EPA recommends a limit of 10 or less per liter in
drinking water). The most famous coliform, Escherichia
coli, is a commensal in our intestine, although some pathogenic strains
exist as well (for instance O157:H7, involved in deadly outbreaks in the US and
Europe). Coliforms behave similarly as fecal pathogenic bacteria, notably regarding
their survival in water. We can thus assume that a sample devoid of coliforms
will also be free of fecal pathogens. Commercially-available tests for coliforms,
such as the culture medium CHROMagar ECC, also permits us to differentiate
between total coliforms and fecal coliforms (mostly E. coli), simply based on the coloration of the bacterial colonies
on agar plates.

At some point during the discussion, I found myself
wondering whether the term ‘coliforms’ could be used as a synonym to Enterobacteria(bacteria from the family
Enterobacteriacaea). I thought that the terms were somewhat equivalent, but I checked
in my BBOM and here’s what I read (pp.906-907):

“Coliforms are defined as facultative aerobic,
gram-negative, nonspore-forming, rod-shaped Bacteria
that ferment lactose with gas formation within 48 hours at 35°C. However, this
is an operational rather than a taxonomic definition, and the coliform group
actually includes a variety of organisms. Most coliforms are members of the
enteric bacterial group. For example, the coliform group includes the organism Escherichia coli, a common intestinal
organism, and the organism Klebsiella
pneumoniae, a less common pathogenic intestinal inhabitant. However, Enterobacter aerogenes, an organism not found
in the enteric group or in the intestine, is also classified as a coliform
because of its fermentative properties.”

So all coliforms belong to the family Enterobacteriacaea,
but not all coliforms inhabit the gut of animals, and not all
Enterobacteriacaea are coliforms. The key point in this quote is that ‘coliform’
is an operational definition. In other
words, we define coliforms as those bacteria that can do so and so (ferment
lactose, etc.). In our example, coliforms happen to coincide with the taxonomic
group of Enterobacteria, but in principle belonging to this group is not a prerequisite
to be classified as a coliform.

Operational definitions are quite widespread in science. For
instance, during my PhD thesis I worked on the effect of dissolved organic
carbon (DOC) on the degradation of hydrocarbon pollutants by bacteria; DOC is simply
defined as all organic matter in solution that can pass a filter of a certain
pore size! Another example from microbiology is the Gram coloration: bacteria
treated with a series of chemical will be stained either red or purple, which
is interpreted as Gram-negative and Gram-positive, respectively.